dry transformer calcs

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websparky

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Hi Guys,

Please help me correctly find the OCD for the primary and secondary or a 45kva transformer.

Primary = 480V
Secondary = 208V

Typical transformer used in a typical commercial application.

Thanks for your help!
 
Re: dry transformer calcs

I?ve never liked Table 450.3(B), because it is not as clear as I could wish. But here?s what I get. I?ll start by presuming you intend to have both primary and secondary protection.

Primary rated current = 45000/831 = 54.15 amps. This is higher than 9 amps, so the maximum is 250% of 54.15, or 135.4 amps. Use the next higher rated breaker of 150.

Secondary rated current = 45000/360 = 125 amps. This is higher than 9 amps, so the maximum is 125% of 125, or 156 amps. Use the next higher rated breaker of 175 amps.

[ October 16, 2003, 06:15 PM: Message edited by: charlie b ]
 
Re: dry transformer calcs

Charlie,

Thanks for the quick response!

I agree, the code book on this is a tad confusing.
However, I came to the same conclusion as you did.
What seemed odd to me, because I don't usually do these calcs, was the 250% note. It just seems too high! I guess I do not fully understand transformers!

When we use these calcs, does the temperature rise significantly?
 
Re: dry transformer calcs

Dave: Primary FLA is 54.2 amps. 52.2 times 1.25 =65.25 A. round up to 70 Amp overcurrent device.

Secondary FLA is 125A times 1.25 = 156.25 Rounded up to next standard size 175A.
 
Re: dry transformer calcs

I stand corrected the primary is 150 Amps.

I was typing as fast as I could, but you guys are good. I made a mistake while in a hurry. :eek:

[ October 16, 2003, 06:32 PM: Message edited by: bennie ]
 
Re: dry transformer calcs

The transformer will not be subjected to load above 125%. If you have both primary and secondary protection, the secondary will trip at 125%. If you have primary only, it is limited to 125%. What I am unsure about is how the primary would ever trip (at 250%) without the secondary having tripped first (at 125%). My only conjecture is that the 250% is there to permit the flow of the higher magnetizing currents that occur during the initial process of energizing a transformer (similar to motor start currents).
 
Re: dry transformer calcs

Charlie,

"My only conjecture is that the 250% is there to permit the flow of the higher magnetizing currents that occur during the initial process of energizing a transformer (similar to motor start currents)."

Wouldn't this only occur at initial power-up?

I have had that happen in the past, but only the first time until the windings are saturated?

Still odd stuff to grasp!
 
Re: dry transformer calcs

Again, it's just a conjecture (I never got along with electromagnetics). But I suspect that a primary set below 125% could trip every time you tried to energize the transformer.
 
Re: dry transformer calcs

After more thought; My 125% calculation is correct unless there is secondary protection. Then the rating can be 250 %.

There once was a requirement that the primary overcurrent device could not exceed the FLA of the primary by more than 250%.

250 times 54.2 = 135.5A, which would be a 125 amp overcurrent device. I don't see that wording anymore.
 
Re: dry transformer calcs

In real life the 250% may be a bit high on the primary, BUT when the transformer is fed from two sources (generator/utility) and the ATS is fast (no built in time delay in transfer, center off) and no inphase monitor the inrush can be quite high, add to this a K rated transformer (which I believe have higher inrush currents, based upon experience) and something above 125% on the primary is necessary, never needed to go all the way to 250% for a typical building XMFR 30-250KVA but have seen this approached

Put two or more transformers on a feeder and a fast ATS, one had better be careful with the feeder OCP and specify an inphase monitor for the ATS.
 
Re: dry transformer calcs

OK now that the maximum OCPD's for the transformer has been discussed, what about the conductor size and protection for the primary and the secondary ? Article 240.
 
Re: dry transformer calcs

Glenn, while we're at it , how do we size a grounded conductor, not a neutral, taking care of say two 175 amp ungrounded conductors? ;)

Roger
 
Re: dry transformer calcs

#4 copper. See 250.24 then 250.66

EDIT: Roger, you didn't mean parrallel when you said (2), did you?

[ October 16, 2003, 10:01 PM: Message edited by: ryan_618 ]
 
Re: dry transformer calcs

Bennie: If you have a 225A 42 circuit panel connected to the 175A breaker, then it's OK to load it up,right? 175A = 63KVA. Does code permit a higher calculated load and is there any time this makes sense? A transformer is essentially 100% rated, while continuous loads need to be set at 125% because of the 80% ratings. If you have HVAC equipment in Las Vegas, should you consider it a continuous load? Even though the code permits more, I use 150A for 45KVA. When a transformer is load protected on the secondary, the primary needs to have short circuit protection only, similar to motors.
 
Re: dry transformer calcs

Ryan, I meant "two" as in two "175 amp" 120 volt ungrounded conductors, not in parallel.

Roger

[ October 16, 2003, 10:22 PM: Message edited by: roger ]
 
Re: dry transformer calcs

I disagree with some of the above methods.

First: conductors are chosen based on the FLC of the transformer not the OCPD device. Transformers (like motors) are considered continuous loads so in this case the 45KVA transformer will need primary conductors of 67.50A and secondary conductors of 156.25A.

Second: the transformer must be protected by a primary OCPD sized at a MAXIMUM of 125% of the primary FLC.
In this case => 54A x 1.25=67.5A, the maximum OCPD is 60A (standard rating).

Third: if you want (or need) to chose a larger primary OCPD you may go as large as 250% MAXIMUM if a secondary OCPD is provide at not more than 125% of the secondary FLC.
In this case, 2.5 x 54 = 135A which gives a primary OCPD of 125A (no up sizing allowed) if a secondary OCPD of 150A is provided (1.25 x 125=156.25A rounded down to nearest standard size).

Fourth: the transformer OCPDs are evaluated for protection of the selected conductors, considering location (tap rules) and ampere ratings.

I do not find any NEC exception for providing short circuit protection only to the primary conductors (as allowed for motor circuits with seperate OL protection). If tap rules are not applicable, the primary conductors may have to be upsized to be compatible with the primary OCPD.
 
Re: dry transformer calcs

Originally posted by jim dungar:In this case, 2.5 x 54 = 135A which gives a primary OCPD of 125A (no up sizing allowed) if a secondary OCPD of 150A is provided (1.25 x 125=156.25A rounded down to nearest standard size).
Click to expand...
I must admit I missed the absence of a ?Note 1? in the ?Primary Protection? column of the ?Primary and Secondary Protection? row of Table 450.3(B). So I agree that 150 is the maximum primary OCPD. But there is a ?Note 1? in the ?Secondary Protection? column, so you can upsize 156 to 175 (no need to downsize to 150).
First: conductors are chosen based on the FLC of the transformer not the OCPD device.
Click to expand...
I don?t agree with this statement. The principal purpose in life of an OCPD is to protect the cable. A breaker set at 150 amps would not protect 67.5 amps worth of primary conductors. I recognize that if the load caused the primary to exceed 125% of the transformer?s rating, the secondary protection (set at 125%) would terminate the event. But a leak to ground or other not-yet-catastrophic failure internal to, or upstream of, the transformer could yield a current between 68 amps and 150 amps, thereby creating the risk of a fire, with no chance that the OCPD would terminate the event.
 
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